Correct interpretation of controller capability flags

[AlCalzone]

Some investigation shows those are quite a mess and don't work as advertised.

The IS_PRIMARY and IS_SIS flags of the GetSerialApiInitData response don't seem to work as advertised.
IS_SIS is true when the current controller is or used to be the primary, and is the SIS. It's false on secondaries that have been assigned the SIS role. It's also incorrect when there are no other nodes on the network. Even when the primary controller has set itself as SIS, IS_SIS == 0.

IS_PRIMARY I cannot really make sense of. It does not indicate a primary controller. IS_REAL_PRIMARY does this.
When IS_REAL_PRIMARY is false, IS_SECONDARY can be used to distinguish true secondary controllers (cannot include/exclude nodes) and inclusion controllers.

The SIS does not have to be the primary controller, unlike the documentation states. After shifting the primary role from the SIS to a secondary, the SIS role stays on the former primary - now inclusion controller?